depends on your car.
Your box cant be bigger than your car.
first find the diminsions of your car.
Make measurements of hieght, depth and width of your trunk.
After you get those numbers, you can most likely make it smaller since you dont want to take up your whole trunk, cuz u might want to get groceries or whatever.

But since they are 12's obvioulsly it has to be taller than 12in's
You will need to find the cutout diameter of the sub, i'm gonna say its prolly 11.125in wide.

For the rest of the box, remember to make sure to take into account for whether or not you will put the top on the top or inside. Because you will have to take into acount the width of the box, otherwise your box will not gotogether the right way and piecew will be to long and to short.